Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Electric mining shovels are commonly used in open-pit mining. Accurate knowledge of the spatial position of such a machine's bucket (or dipper) is important, and finds application in several operational contexts. Examples include:                Control strategies for avoiding collisions between the bucket and obstacles, including self-collisions.        Monitoring technologies, such payload weighing systems and ore grade monitoring tools.        
Accurate determination of bucket position requires calibration of the sensors used to measure swing, crowd and hoist motions. In practice, these sensors are typically angular position sensors (e.g. rotary resolvers) attached to actuators (e.g. electric motors) of the machine's motion axes.
The bucket position can be established by using these sensor readings in conjunction with a so-called forward kinematic map that relates the positions of the motors to the positions of the bucket. Calibration amounts to determining offsets for each resolver. When these offsets are added to the measured sensor values, they accurately establish actual bucket position through the forward kinematic map.
Calibration of the swing resolver can be realized using a secondary device that indicates when the swing axis is at zero position. Such devices can be readily constructed by people skilled in the field. The hoist and crowd motions are normally coupled and a more sophisticated approach is needed.
The control systems of modern mining shovels provide procedures for calibrating hoist and crowd resolver offsets. These procedures require that the operator position the bucket at a sequence of specified locations. The process relies on operator judgement and in practice the determination of offsets using these methods is inaccurate and imprecise. Recalibration must be performed at frequent intervals, for example when the bucket or the hoist ropes are changed.
Wauge (see Wauge, D. 2007. Payload Estimation for Electric Mining Shovels, PhD Thesis, The University of Queensland, Australia) describes a multi-point photometric method to calibrate the offsets of the hoist and crowd resolvers. This technique generates accurate calibration results, but is labour intensive and not appropriate in a production environment.